1. Field of the Invention
The present invention relates to three-way catalysts which are particularly effective in treating the exhaust gases of internal combustion engines. The present invention also relates to the use of such catalysts in the treatment of internal combustion engine exhaust gases.
The expression "three-way catalysts" for the purposes of the present invention is taken as meaning catalysts which effect the oxidation, in particular, of the carbon monoxide and hydrocarbons present in internal combustion engine exhaust gases, and also the simultaneous reduction, in particular, of the nitrogen oxides present in such gases.
2. Description of the Prior Art
In gasoline fueled internal combustion engines, the composition of the exhaust gases generated thereby can be adjusted to attain a stoichiometric equilibrium such that the catalytic oxidation and reduction of its various constituents result in water, carbon dioxide and nitrogen. The methods generally used for adjusting the composition of the exhaust gas in order to achieve this stoichiometry include, in particular, the continuous adjustment of the air/fuel ratio at the engine intake, and/or the introduction of additional oxygen upstream of the catalyst. The composition of the exhaust gas can thus be varied within seconds from a composition containing a relative excess of oxidizing compounds (a so-called "lean" adjustment) to a composition containing an excess of reducing compounds (a so-called "rich" adjustment), and vice versa.
In particular, the so-called "lean" adjustment is such that the amounts of oxygen and nitrogen oxide present are greater than those required to oxidize the carbon monoxide, hydrocarbons and hydrogen present. Conversely, the so-called "rich" adjustment is such that the amounts of carbon monoxide, hydrocarbons and hydrogen present are greater than those required to reduce the oxygen and nitrogen oxides present.
Catalysts for the treatment of exhaust gases have been proposed. For example, in the Journal of Catalysis, Vol. 56, 321 (1979) and the 6th North American Congress of the Catalysis Society--Chicago--March 1979--Communication H.sub.4, three-way catalysts are disclosed which contain cerium oxide and precious metals as the active phase.
Japanese Application No. J53,062-777 published the 28th week of 1977, discloses three-way catalysts which employ iron oxide and precious metals as the active phase.
Japanese Application No. 76/49,190 J51, 049-190 published the 24th week of 1976 discloses three-way catalysts which use lanthanum, barium, copper, rhodium, platinum and/or palladium in the active phase.
Japanese Application No. 74/23,790 J49, 023-790 published the 39th week of 1974 discloses three-way catalysts which contain ruthenium, palladium and, if appropriate, a metal chosen from among Nb, Ce, Ti, Fe, Ni and Cu, as the active phase.
Such catalysts, however, prove to have an inadequate initial activity and an inadequate stability over time to satisfy the current anti-pollution requirements.
Accordingly, it is an object of the present invention to provide a novel and improved catalyst which is active in the treatment of internal combustion engine exhaust gases.
It is another object of the present invention to provide a catalyst which exhibits an improved initial activity in the treatment of internal combustion engine exhaust gases.
It is still another object of the present invention to provide a stable catalyst for the treatment of internal combustion engine exhaust gases.
Yet another object of the present invention is to provide a process for the treatment of internal combustion engine exhaust gases which utilizes a catalyst of improved initial activity and stability.
These and other objects, as well as the scope, nature and utilization of the invention, will be apparent to those skilled in the art from the following description and the appended claims.